where can I download cycles as an addon and why does blender not run?
Cycles cannot be downloaded as an add-on and installed in an older version of Blender - it's too tightly integrated for that.

In case of future googlers coming across this question, Cycles can be used standalone, but that's not really related to your problem.

If you believe blender should be able to run on your computer (your specific graphics card supports OpenGL 2.1, google the model name), then you should report a bug and let the developers help you find out what's wrong. When describing your problem, be very specific about what's wrong, what error messages there are, your hardware, etc.